This invention relates to glass breakage detectors, and in particular to glass breakage detectors that utilize digital signal processing to determine if the signals produced by an acoustic transducer are the result of glass breakage. The term glass breakage as used herein refers to the breakage of framed glass, such as windows or doors, and not to the breakage of glass items, such as drinking glasses and the like.
Home and commercial security systems commonly use glass breakage detectors to detect the presence of an intruder. When an intruder breaks a window to enter the premises, the glass breakage detector detects the breakage of glass and an alarm is sounded. Glass breakage detectors with acoustic transducers monitor the sounds in the local environment. Acoustic glass breakage detectors of the prior art monitor the amplitude of the sound at frequencies that are typically associated with glass breakage to determine if the received sound is a result of glass breakage.
Acoustic detectors available today have a tendency to generate false alarms on other noises found in the home or business such as the shaking of keys, slamming of a file drawer, clapping of hands, etc. In order to reduce the incidence of false alarms, acoustic detectors of the prior art use multiple analog filters in order to selectively pass only frequencies associated with the breakage of glass. A glass breakage detector which comprises multiple hardware filters and which monitors the amplitude of the filtered signals is disclosed in U.S. Pat. No. 5,323,141, which is incorporated by reference herein. The amplitudes within the chosen bands are compared to a predetermined threshold value in order to detect the glass breakage.
Another glass breakage detector of the prior art, disclosed in U.S. Pat. No. 5,552,770, recognizes temporal events that typically accompany glass breakage. The high frequency sound of the impact is detected, followed by low frequencies caused by flexing of the glass due to the impact, and high frequencies again when the glass breaks by shattering. An alarm signal is issued by the glass breakage detector only when the detected low frequencies last for a predetermined minimum duration beginning not before the first detection of high frequencies. This glass breakage detector uses hardware filters and timing circuits to detect the glass breakage. Such a detector is an improvement over other acoustic detectors, but the improvement comes at the cost of extra hardware circuits. The size and cost of the hardware places limits on the number of filters a detector can have.
Acoustic detectors of today need adjustments during installation to work properly in the different environments in which they are installed. Acoustic waves resulting from a glass breakage event are a function of glass type, window frame configuration, room acoustics, and distance from the window. A small change in distance between the window and the transducer results in a large change in the received sound level. A range adjustment allows an installer to change the sensitivity of the acoustic detector to adapt it to its placement in the room. This adjustment may sometimes cause the detector to miss a glass breakage event. When the range setting is adjusted improperly by the installer, the breaking of a window may not exceed the detector's threshold. To compound the problem, the installer remains unaware of the improper installation since a typical installation generally does not involve breaking an actual window. Some manufacturers design acoustic detectors with high gain amplifiers to ensure detection of glass breakage from the maximum recommended distance; this, however, results in amplifier saturation when the detector is mounted near the glass. It would be advantageous to have an acoustic detector which operates reliably over a vast range of sound levels thereby reducing installation errors.
In many environments, sounds specific to that environment create false alarms that are not easily discriminated against by acoustic detectors available today. In these environments, it would be advantageous to customize the detector by analyzing the sounds produced by the specific false alarm and modifying the detector to discriminate against that sound. It would also be advantageous to store the features of the sounds that generate an alarm so that later analysis of these features is possible.
It is therefore an object of the present invention to provide a glass breakage detection device with increased sensitivity without increased false alarms.
It is a further object of the present invention to provide a glass breakage detection device that detects a plurality of the features generated during a glass breakage event.
It is a further object of the present invention to provide a glass breakage detection device that may be adapted to detect a simulated glass breakage event during installation.
It is a further object of the present invention to provide a glass breakage detection device with the ability to be modified to include updated technology or to be customized for a particular environment.
It is a further object of the present invention to provide a glass breakage detection device that compensates for the characteristics of the room in which it is mounted.
It is a further object of the present invention to provide a device that corrects the front end offset errors of the glass breakage detection device.
It is a further object of the present invention to provide a device that transmits and stores features for computer analysis.